1. Field of the Invention
The present invention relates to a manufacturing method for a semiconductor device and to a semiconductor device. More particularly, it pertains to a manufacturing method for a semiconductor device in which, in a die-bonding step, one of the steps of assembling the semiconductor device, a solder foil is used to bond a die to a die pad through a liquid phase, diffusing bonding, and it also pertains to a semiconductor device manufactured by such a method.
2. Description of the Related Art
FIG. 4 is a sectional side elevation schematically showing a die of a conventional semiconductor device before the mounting of the die. As shown in FIG. 4, the die 1 of a semiconductor element is bonded to a die pad 3 through a soldering material 2. The soldering material 2 is a foil-like bonding material including a diffused layer 4, which is an intermediate layer having a relatively high melting point, such as 95 wt. % Pb-5 wt. % Sn or Sn and diffusing layers 5, which are outer layers having a low melting point, relative to the diffused layer 4, such as 65 wt. % Sn-35 wt. % Pb when the diffused layer 4 is 95 wt. % Pb-5 wt. % Sn or 95 wt. % Pb-5 wt. % Sn when the diffused layer 4 is Sn. The diffusing layers 5 are laminated on diffused layer 4.
The conventional semiconductor device is constructed as described above. When the die 1 is bonded to the die pad 3, the diffusing layers 5 serve as initial bonding layers because the reverse surface of the die 1 and the obverse surface of the die pad 3 are heat-treated. In other words, because of the heat treatment, first, the diffusing layers 5, having a low melting point, melt, and then the soldering material 2 is bonded to the die 1 and the die pad 3. When these components are maintained at a predetermined temperature, metal in the diffusing layers 5 diffuses into the diffused layer 4, and the composition of the soldering material 2 becomes uniform, thus gaining desired characteristics. The die 1 and the die pad 3 can be bonded together in this way.
In the semiconductor device as described above, in order to fully obtain initial bonding, since the diffused layer 4 is covered with the diffusing layers 5, one diffusing layer 5 must be bonded to the reverse surface of the die 1 and the other diffusing layer 5 must at the same time be bonded to the obverse surface of the die pad 3. The soldering material 2 must be thicker than a predetermined value in order to fully obtain the initial bonding and to compensate for the effect on the configurations of the die 1 and the die pad 3. After the initial bonding, the metal in the diffusing layers 5 diffuses further into the diffused layer 4 because of the heat treatment. While the metal is diffusing, when part of one diffusing layer 5 and part of the other diffusing layer 5 are superposed on each other at the center of the diffused layer 4, the concentration gradients of the compositions of the diffused layer 4 and the diffusing layers 5 become smaller. Therefore, the diffusion rate of the metal tends to decrease. For the above reasons, it takes an enormous amount of time for the soldering material 2 to be heated before the composition of the material 2 becomes uniform.